A surgical tool is known (for example the model number 296-17-40 manufactured by the assignee of the present invention) in which a worm is rotatably reversibly driven by an input shaft and in turn drives a worm gear for driving a tool, in which the worm shaft and input shaft are disposed end to end with their rotational axes at an obtuse angle and the input shaft drives the worm shaft through bevel gears mounted on the adjacent ends of the shafts. In a prior surgical tool provided with such an angle drive, rotational driving of the worm gear sets up axial thrust forces tending either to crush the bevel gears axially together or to axially separate them, depending on the direction of rotation of the worm. In a prior design, the axial forces tending to crush the bevel gears together are countered by a rigid outwardly extending annular shoulder on the worm shaft which, through a surrounding axial thrust bearing, is backed by a portion of the surgical tool housing. Unfortunately, the thrust bearing must be relatively large in diameter, compared to the shaft diameter, and hence is bulky and expensive. Further, the bearing must be capable of handling loads which, in view of its large diameter, are imposed at relatively high circumferential speeds. Under these conditions, a multielement, so-called low friction bearing (multiple ball or roller bearing) is called for, thus increasing the cost of the tool. Substitution of a simple single element face to face sliding bearing risks bearing heating and relatively rapid wear of the axially opposed, relatively rotating rubbing surfaces of the axially opposed, annular bearing faces.
Further, radial thrust bearing means are required, in order to maintain the rotational axis of the worm shaft fixed with respect to the rotational axis of the input shaft which drives it. Such increases the cost and complication of the above-described axle thrust bearing arrangement.
Further, relatively complex machining of the interior of the housing must be carried out with a relatively high degree of accuracy to correctly locate the mentioned thrust bearing coacting between the worm shaft and housing.
In addition, despite care taken to maintain close tolerances in the location of the bearing members on the housing and on the worm shaft, it is difficult to achieve, and more difficult to maintain over time, optimum clearances between coacting portions of the bevel gears on the adjacent ends of the two shafts, such that slight misalignment may readily occur, leading to excessive friction between, and wear on, the meshing bevel gears.
In addition, since the worm shaft may be driven in either rotational direction, the bearings must be arranged to handle axial thrust loads in either axial direction along the worm shaft, as well as radial loads thereon. This is particularly true in view of the driving by the worm shaft of a worm gear transverse thereto.
Accordingly, the objects and purposes of this invention include provision of an angle drive for a surgical power tool which is intended to overcome the above-described drawbacks associated with known obtuse angle drive surgical tools. The objects of the present invention further include provision of a surgical tool angle drive which reacts against the axial and radial loads applied to the shafts and bevel gears disposed therebetween while maintaining accurate alignment and positioning of the mesh of the bevel gears, which provides a low friction rolling contact having a circular line of contact with each shaft rather than the prior rubbing surface contact or complex multiple ball or roller bearing of prior devices, and which handles thrust loads on the rotating worm shaft at a relatively small bearing diameter for reducing tangential surface speeds at the bearings. The objects of the invention further include provision of a bearing structure which is mechanically very simple, which requires only relatively minor and easily implemented machining of the adjacent shaft ends, which eliminates the need for machining within the housing to accommodate the bearing mount surrounding the worm shaft, and which tends to allow the bevel gears on the respective shaft ends to float with respect to each other and thereby maintain proper clearances therebetween for a long service life and efficient transfer of motion.
Other objects and purposes of the invention will be apparent to persons of ordinary skill in this art upon reading the following description and inspecting the accompanying drawings.